Ion beam induced luminescence spectra of lithium fluoride at high-and low-temperature

Abstract

A new ion beam induced luminescence (IBIL) measuring setup, equipped with a custom-made heating/cooling sample stage (the attainable temperature ranges from 80 K to 900 K), has been established on the GIC4117 tandem accelerator in Beijing Normal University. As the yield of back scattering ions is proportional to the beam flux, an Au-Si surface barrier detector is employed to count the back scattering ions synchronously with collecting the IBIL spectra under the multi-channel scaler (MCS) mode of the multichannel analyzer, making it possible to online monitor the beam current. Then, the yield of back scattering ions is used to correct the intensity of the IBIL spectrum and calculate the ion fluence, for eliminating the influence of the beam current fluctuation. IBIL spectra of pure lithium fluoride (LiF) at different temperatures (100, 200, 290, 450, 550 K) under the 2 MeV H+ irradiation are acquired and the significant influence of temperature on luminescence centers is observed. The emission bands relating to exciton recombination (296 and 340 nm) and impurities (400 nm) are more prominent at low temperatures and present quite lower intensities at high temperatures. Moreover, these luminescent intensities decay with ion fluence increase obviously at high temperatures after initially increasing in the early period of irradiation. The initial increase of the disturbed exciton peak at 296 nm can be attributed to the strained bonds produced by nuclear elastic scattering at a low fluence, which was not observed in previous IBIL measurements under high ionization energy density or high ion beam flux. This observed increase indicates that the emission feature may also originate from the emitting centers relating to point defects, not just from exciton transition near lattice or impurities. The luminescent intensities of F2 color centers (peaked at 670 nm) are dominant at all temperatures, while the luminescent intensities of F3+ color centers (peaked at 540 nm) are not obvious at low temperatures and the luminescent intensities of F3-/F2+ color centers (peaked at 880 nm) are weak at high temperatures. The luminescent intensities of these F-type centers reach saturated values at lower fluences at high temperatures. The different evolution behaviors under different temperatures can be due to the influence of temperature on the vacancy migration rate and the non-radiative recombination. In addition, the surface charge accumulation may lead to the luminescent intensities of color centers reaching saturated values at higher fluences, compared with the previous IBIL measurements of LiF. The self-absorption effect would reduce the intensities of F3+ color centers because of the absorption of F-type centers at low temperatures, while the effect is weak at high temperatures due to the degradation of F-type centers.